Testing the 6AV5GA sweep tube:
This testing started as a thread on the diyAudio forums.
This story began several years ago with rumors that the 6AV5 was a possible substitute for the 6B4. Since the 6B4 is an octal version of the 6A3, which is a 6 volt version of the 2A3. some people were calling the 6AV5 a modern version of the 2A3. There was even an " urban legend" that in order to fulfill a government contract a major tube manufacturer made a large run of 6AV5's with the bases rewired as triodes and labeled them 6B4GA's. Several reports of 6B4's with straight sided glass envelopes and multiple grids were circulating on the internet, but no one had any pictures or other hard evidence.
Apparently this question had been asked and experiments proposed on one of the other audio forums, but no one actually tested any tubes. Someone asked on diyAudio if this story was even possible since the 6AV5 has a maximum screen voltage of 175 volts. How could you triode wire it and apply 275 (or more) volts? Well I took this as a challenge and decided to compare the 6AV5 to the 6B4. How?
First, I took a freshly built TubelabSE board, and set it up on the bench. The rectifier tube was removed and the variable power supply seen behind the board was connected. This allowed variable B+ voltage. I connected up a pair of One Electron UBT-3 output transformers. The amp is seen here operating with 300B's to verify proper operation, and to take reference measurements.
I tested the setup with 300B's, which provided the expected 9 watts at 5% distortion (my common comparison point) with the power supply set at 400 volts. Listening tests proved that this setup sounded the same as an unmodified amp. Then I gathered up a few tubes that I wanted to experiment with. The box to the left of the voltmeter is full of 6B4's.
Next test was a pair of NOS Sylvania 6B4's. I wired the 6B4's into the amp using clip leads. A separate power supply was added to power the filaments. Max voltage spec for the 6B4 is 325 Volts, so that is where I set the power supply. Power (at 5%) was just over 3 watts. They sounded like 2A3's, no surprise here. The bass was not as strong as the 300B's and some of the clarity seemed to be missing. I have used 2A3's in these amplifiers, and these sounded similar. I tried other used 6B4's and the sound was similar. I think that the Sylvanias were the best that I tried.
Now for the 6AV5's. (NOS but unboxed and dirty GE's) I wired them up in
triode connection, flipped the B+ switch, the current meter slammed to the
right, and interference showed up on a nearby TV set. These puppies really
like to oscillate. I re did all of the wiring, added stoppers right at the
socket for G1, and G2. The oscillation was gone. I started at 250 volts.
The tube worked, and biased up similar to the 6B4. It didn't sound half
bad, in fact I think that it sounded better than the 6B4, but not as good
as the 300B.
Time for more
voltage, 325. These things ROCK. I am now running 325 volts at 75mA which
is over 24 watts dissipation. I ran it this way for an hour. There is no
death glow even with the room lights off, the screen wires are not
glowing. This is over twice the recommended daily allowance of power
flowing through this little guy, and it is still smiling. I made a power
measurement and I am getting 12 watts at 5%. Distortion at 5 watts is
under 1%. The sound is pretty good. Loud, dynamic, but not as detailed as
a real triode. I tried putting the 8 ohm load on the 16 ohm tap to present
a 1.5 K ohm load to the tube. Power rose to 15 watts, distortion went up
at 5 watts to over 2%, and it didn't sound as good.
OK, I still haven't killed it yet, so I turned the power supply up to 380 volts. Current is now 105 mA. (bias was never readjusted from the initial settings). This is almost 40 watts dissipation (some voltage is dropped in the OPT) the red glow is beginning to appear on the plate, however the screen grid wires are NOT glowing. This should alleviate any fears about violating the 175 volt screen rating. Power output is 17 Watts RMS at 5%. This amp now sounds loud, and dynamic, but at this level there is some oscillation seen on the TV on loud bass, which shows up in the sound. With everything connected up using clip leads, this is not unexpected.
This was as far as I took the experiments on that particular thread, I went on to try other sweep tubes in triode wired mode. Some worked well and some did not like having their screen voltage rating violated. After that I started testing other tubes in my collection.
Note: Since I did these experiments, and posted the results on the forums, I have actually SEEN and TOUCHED the "urban legend". The tubes DO exist. The one I saw was a Sylvania. It was clearly marked 6B4GA, and it looked IDENTICAL in construction to the Sylvania 6AV5's that I have. The owner said he also has some of these tubes that are Philips branded. He said that Angela Instruments was selling them. If the rest of the story is true, and Sylvania (or Philips) really sold 6AV5's as 6B4's, does this mean that the tubes are identical? NO! This means that the tubes were similar enough to be interchangeable IN THE ORIGINAL APPLICATION. There is no way that the two can ever be identical. In my listening tests there were obvious differences between the two in sound. The 6AV5 had far better bass, better dynamics, and was capable of far more power. The 6AV5 had a lower plate resistance, and was capable of passing a lot more peak current than any similar sized DHT.
The story does not end there. I heard about some extremely low cost output transformers on another thread ( Cheap SE output transformers ), Edcor XSE15-8-5K, that were getting good reports, so I ordered a pair. After testing them with the usual DHT's and the like, I decided to go back to the 6AV5's. To cover the testing and torturing of this combination, I started another thread. The Edcor meets the 6AV5
The original test setup was long gone, so I simply repeated the procedure. Only this time the only TubelabSE I had was my 300B amp in the Lexan case. I have experimented on it before, so I offered it up to science again.
Here is the Lexan amp with 6AV5's connected up using clip leads. This time the clip leads are twisted together, and the whole setup is a lot neater than before. No more oscillation!
The first test was ran at the rated 11 watts dissipation, 300 volts and 35 mA. The amp sounded OK but kind of subdued. In my usual style I turned up the power. Much to my surprise they started to exhibit the death glow at about 15 watts. This is not what I saw before. These were NOS tubes, not dirty old ones. So I found the old tubes that I had used before. OK now I can crank up the power. I was running 350 volts at 60 mA with no glow. They sounded nice, good bass, clear sparkling mids and crystal clear highs. I listened for two hours with a wide variety of music. It sounded good, but not as quite as good as the Edcor with the 300B.
This was puzzling, the first pair of 6AV5's (GE's) were showing no signs of distress at 24 watts dissipation, while the second set (RCA's) was beginning to exhibit the glow of death at 15 watts. Clearly all 6AV5's are not created equal.
I went through my entire collection of sweep tubes and pulled out all of the 6AV5's. Upon further investigation I found at least 5 different internal constructions for the 6AV5GA. The NOS RCA's that I tested had large slots in the radiating fins at the point where the fins attach to the plate. This would limit the thermal conductivity. What were they thinking? I also found RCA's without these slots. I have some Sylvanias, and several other flavors.
When I was at the Orlando hamfest (February) I went searching for 6AV5's. I bought every one that I could get for under $4 each. Most were $3. I got about 20. I found 6 distinct types of 6AV5 tubes. Two types of RCA's (with and without the slots in the radiating fins), two GE's, a Raytheon, and the Sylvania. There were other brands but they appeared to be rebranded versions of the types listed above.
I have just finished torturing (I mean testing) each
type. The dissipation figures that I use are total (plate and screen) tube
current times actual plate to cathode voltage. All tubes were tested at
300 volts triode strapped unless otherwise noted. All of this testing was
done at very low volume (milliwatts), this is worst case for dissipation.
Sherri has friends over, so I can't crank it up loud anyway. The results
are listed below.
This photo was
taken of a Sylvania 6AV5 at 315 volts and 130mA, this is 41 watts. Notice
the even glow across the entire radiating fin. There are no hot spots.
This shows good thermal design.
If you intend to build a 6AV5 amplifier and keep it around for a while, don't push the tubes too hard unless you plan to change them often. Transistors just blow up when you push them too hard. Tubes can be operated above the maximum dissipation rating at the expense of shorter life. It is not uncommon for a guitar player who plays 4 or 5 two hour shows a week to need new tubes in six months or less. NOS tubes seem to live longer than many current production tubes due to better vacuum, and most sweep tubes were over designed due to the severe service they were in. 6AV5's, 12AV5's and 25AV5's are cheap (for now) so if you plan to push them hard get some spares. My rule of thumb is to turn off the room lights and look for red spots on the plate, and glowing grid wires. Back off the bias until this goes away.
Some of these tubes seem more sensitive to voltage than others. I am using a variable power supply so I can watch the current as I turn up the voltage. All triode connected sweep tubes will have a point where the current starts to rise quickly. Operation at or above this point could cause an uncontrolled runaway condition. I believe this is because we are way above the screen voltage rating. This happens at about 295 volts on some 6AV5's while others are happy with 350 volts. If you are going anywhere near this you should put a 100 (or 125) milliamp fuse in the cathode leg. It may save your transformers if a tube fails.
I decided to perform some basic amplifier measurements on the Edcor - 6AV5 combination. I discovered (while playing around with the 6DN7 and 6EM7) that the Edcor transformer really likes cathode feedback, so I tested that too. I set an arbitrary 5% test limit for distortion.
In triode strapped mode I got 4.75 watts at
5% with 300 volts and 60 mA. Adding cathode feedback allows 8 watts at 5%
using the same voltage and current. Distortion at all power levels is
Every once in a great
while you stumble across a combination that works way too good considering
what it is made of. I knew that I was on to something last night when I
tried the 6AV5's through the Edcors in UL mode with cathode feedback. The
measured performance was good. Well when I got home from work today Sherri
was not home, so I connected up the speakers and cranked it up. This thing
positively ROCKS, I played a lot of different music, the CD's that I have
previously identified for their abilities in finding amplifier flaws.
Complex music pushed well into clipping still sounds good, there is NO
intermodulation from heavy bass guitar over a nylon stringed acoustical
guitar. Voices and saxes do not get affected at all by drums or bass even
with the volume knob all the way up. I can tell by the bouncing current
meter that I am not in class A any more, but distortion can only be heard
on a few heavy bass notes, and it doesn't sound gross.
Frequency response plot at 8 watts. The vertical scale is 1db per division.
Note: Since this was written, this eventually evolved into the SimpleSE. Due to many e-mails and forum posts, the 6AV5 was replaced with a 6L6 or an EL-34 in that design. That decision came from two problems. The 6AV5 can not be easily found in Europe and Asia, and there is a wide variation in the dissipation capabilities of the various brands.
The 6AV5 story is not over yet. During the testing of the 6AV5 in SE mode one of the forum members mentioned the outrageous power that he was getting by using sweep tubes in screen driven push pull. I just had to try it.
Here is the close up of the amp running at 80 watts. It had been running for over an hour when this picture was taken.
This amplifier uses screen drive
(no signal on the control grid). Screen drive works well with sweep tubes
because they have a high gain screen grid. This is why the screen grid
voltage spec is so low. Screen drive does not work well with normal audio
tubes that can handle a lot of screen voltage. I plan to take this design
further at a later date. I just don't have time now.